Mobile phones have built-in vibrators that produce mechanical vibrations, which are intended to be felt by a user of the mobile phone as an alert or feedback mechanism. The vibrator is part of what is referred to as a haptic alert device or haptic feedback device. In a linear vibrator, a spring-loaded weight is attached to a moving element of a linear electric motor. The motor drives the attached weight back-and-forth, in response to a sinusoidal input drive current. The frequency of the drive current is controlled so that it coincides with a resonant frequency of the spring, weight and motor combination. This enables the driven weight to produce strong vibrations.